- Email: [email protected]
Bilateral Leydig Cell Tumor in Stallion
REFEREED
CASE REPORT Bilateral Leydig Cell Tumor in Stallion
C.M. Melo, MS,a F.O. Papa, PhD,a N.C. Prestes, PhD,a M.A. Alvarenga, PhD,a and R. Laufer-Amorim, PhDb ABSTRACT The authors report a rare case of bilateral Leydig cell tumor in a noncryptorchid stallion, describing the gross and microscopic findings. An 8-year-old Appaloosa stallion was examined for a unilateral enlargement of the testis associated with discomfort during palpation and signs of colic. General health conditions were good. Fine-needle aspiration cytology of the testis provided the diagnosis of bilateral Leydig cell tumor. Bilateral orchiectomy was performed and the testes were submitted to histopathologic examination that confirmed the diagnosis of Leydig cell tumor. Keywords: Testis; Interstitial cell tumor; Stallion
INTRODUCTION Testicular tumors are uncommon in stallions, probably because most male horses are castrated at an early age, making it difficult to predict the true incidence of this pathologic condition.1,2 The description of endocrine dysfunction, malignancy, and relationship with cryptorchid testicle is based on other species commonly affected by this condition, such as dogs and humans.2,3 Testicular neoplasms are rare and can be classified as primary, which are the most common tumors, and secondary.1,2 In general, the neoplasias are classified as germinative, comprising seminoma, teratoma, teratocarcinoma, and embrionary carcinoma, or nongerminative: leydigocitoma, Sertoli cell tumor, and lipomas.4 Seminoma is by far the most common testicular tumor of the mature stallion.1,3,4 Fine-needle aspiration biopsy can help to differentiate inflammatory from noninflammatory disease and may allow diagnosis of neoplasia by an experienced cytologist.1 Fine-needle aspiration biopsy has proven to be a simple and minimally invasive diagnostic procedure.5 Testicular ultrasonography is a valuable tool to differentiate intratesticular or extratesticular masses in humans.6 In
stallions, little information is available about use of ultrasonography to determine the type of tumor; consequently a histopathologic examination is necessary to confirm the diagnosis. Tumoral images are variable; thus, ultrasonography will help to identify discrete masses and characterize their density.7 Normal parenchyma should be echoic, whereas focal changes of echogenicity are often associated with a tumor, hematoma, abscess, or local inflammation.8 Gelberg and McEntee9 presented data from nine stallions with Leydig cell tumor. Two of them showed aggressive behavior. Thus, these authors suggested that, in some cases, equine Leydig cell tumor might lead to abnormal steroidogenesis. As reviewed by Papatsoris et al10 in humans, 20% of the cases of Leydig cell tumor demonstrated hormonal activity, including an increase in estradiol and a decrease in testosterone serum levels. Treatment consists of castration using closed techniques, and both testicle and spermatic cord should be removed.1,2 The aim of the current study was to describe the presence of a bilateral Leydig cell tumor in a noncryptorchid stallion and its gross and microscopic findings.
CLINICAL HISTORY
From the Department of Animal Reproduction and Veterinary Radiology—UNESP/Sa˜o Paulo State Universitya; and the Department of Veterinary Clinic—UNESP/Sa˜o Paulo State Universityb; Sa˜o Paulo, Brazil. Reprint requests: C. M. Melo, FMVZ—Department of Animal Reproduction and Veterinary Radiology, Distrito de Rubia˜o Ju´nior s/n8, 18618-000, Botucatu, SP, Brazil. 0737-0806/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2007.08.002
An 8-year-old Appaloosa stallion was presented to the Veterinary Teaching Hospital, Sa˜o Paulo State University, Botucatu, Sao Paulo, for investigation of discomfort during breeding and unilateral enlarged testis for 1 year (Fig. 1). The referring veterinarian had treated the stallion with metronidazole, sulfa, and trimethoprim because of suspicion of an infectious orchitis. He also reported that the stallion had preliminary signs of colic. Standard physical examination of the stallion showed good appearance and all physiologic parameters in order. On testicular palpation, the stallion exhibited pain; testicular texture varied from fibroelastic to firm. Testicles had the following sizes: 12.0 8.0 8.0 cm (left testis) and 6.0 5.0 5.0 cm (right testis) for length, height, and width, respectively. The ultrasound evaluation showed hypoechogenic areas similar to abscess (Fig. 2), which were confirmed at macroscopic examination of testis after surgery (Figs. 3, 4). During semen collection, no abnormal sexual or aggressive behavior was observed and no signs of pain were manifested during erection until ejaculation. Semen analysis showed oligoasthenospermia. Aspirative cytology was performed with a fine needle and a bilateral Leydig cell tumor was diagnosed.
450
Journal of Equine Veterinary Science Vol 27, No 10 (2007)
CM Melo et al Vol 27, No 10 (2007)
Figure 1. Testicular appearance during physical examination.
Figure 2. Ultrasound image of the left and right testes showing the abscess.
According to Eisenhauer et al,11 luteinizing hormone (LH) stimulates production and release of estrogen and testosterone from the Leydig cells. To investigate the influence of the tumor on hormonal profile, blood samples were collected for hormone assays. The human chorionic gonadotropin (hCG) challenge is a test to investigate testicular function of stallions and a diagnostic tool to test the secretory capacity of Leydig cells.12 Because LH promotes spermatogenesis by stimulating Leydig cells to produce testosterone, the stallion received a 6,000-IU hCG intramuscular injection. Jugular blood was collected by venipuncture into Vacutainer tubes before hCG treatment and 48 hours after hCG. The blood was centrifuged for 5 minutes at 1,500g. Serum was stored at –208C until being assayed for concentrations of estrogen and testosterone, performed by BET Labs (Lexington, KY). Hormonal values are presented in Table 1.
451
Figure 3. Left and right testes after orchiectomy.
Figure 4. Left and right testes sectioned after the orchiectomy.
Gross Findings After orchiectomy, testicles were sectioned for macroscopic evaluation. Both testes presented circumscribed masses. On gross examination, both testes presented circumscribed encapsulated tissue with a different aspect from normal parenchyma, containing cystic areas filled with colorless yellow fluid. The tumor was hazel in color and had a smooth texture (Fig. 4).
Microscopic Findings The testes were submitted to histopathology. Histologic examination showed polyhedral cells with granular eosinophilic cytoplasm and vacuolar spaces. The nuclei were small, spherical, and basophilic with scant supporting stromal tissue, presenting a necrotic center with a suppurative inflammatory process (Fig. 5).
CM Melo et al Vol 27, No 10 (2007)
452
Table 1. Estrogen and Testosterone Levels at 0 and 48 Hours after hCG Administration T[0 Hours
T [ 48 Hours
Total estrogen (pg/ml) 338.5 183.9 Testosterone (pg/ml) 2,015.2 3,465.4 Normal values in horses (B.E.T. Laboratories) Total estrogen (pg/ml) Stallion 150 400 pg/ml Gelding < 50 pg/ml Testosterone (pg/ml) Stallion 500 2,000 pg/ml Gelding < 50 pg/ml
Figure 5. Photomicrograph of a section of the testis from a horse with testicular interstitial cell tumor. Notice polyhedral cells with granular eosinophilic cytoplasm and discrete cytoplasmic borders with small, spherical and basophilic nuclei (hematoxylin-eosin, original magnification 400).
current report, the stallion presented some episodes of colic without a clinical cause, similar to the case described by May et al.18 The stallion had no discomfort during semen collection; sperm analysis showed oligoasthenospermia. Papatsoris et al10 suggested that human semen analysis may show oligozoospermia, cryptozoospermia, or azoospermia. Orchiectomy was performed on the day after treatment with hCG, and blood was collected for estrogen and testosterone dosage at 0 and 48 hours after hCG administration. Serum concentration of estrogen was normal both before and 48 hours after hCG injection. Testosterone levels had a significant increase at 48 hours after hCG challenge. However, as expected, a decrease to basal levels (<50 pg/ ml) was observed in testosterone and estrogen levels at 96 hours after removal of testicles. This finding demonstrates that the tumor did not affect responsiveness to LH stimulation by increasing the concentration of testosterone. The use of fine-needle aspiration biopsy was as efficient as histologic evaluation, which may lead to the conclusion that this examination should be performed in stallions with a history of infertility, especially associated with an abnormal testis. Finally, the current case was the first report of a bilateral Leydig cell tumor in a stallion with both descended testes. The current report also evaluates endocrinology of the tumor, which has not been described by other authors.3,9,18,19 The evaluation and description of Leydig cell tumors in stallions should be performed whenever it is possible; it can contribute to our knowledge and understanding of this tumor, especially endocrine evaluations. ACKNOWLEDGMENTS The authors thank BET Labs for the hormonal evaluation.
REFERENCES 1. Schumacher J, Varner DD. Neoplasia of the stallion’s reproductive
DISCUSSION AND CONCLUSION Primary testicular tumors are relatively common in old dogs, less common in aged bulls, and rare in other species. These tumors can be of three types: interstitial Leydig cell, Sertoli cell, and germinative cells. These neoplasms are observed in sexually mature animals or older males, as in interstitial cell tumors in dog.13 Testicular neoplasms are rare in horses,14-17 although it is difficult to predict or estimate their occurrence, because most animals are castrated young.3 Cryptorchidism may be a contributing factor in the formation of Leydig cell tumors in stallions. Gelberg and McEntee9 observed the occurrence of this tumor in the abdominal testes of 8 of 9 cryptorchid stallions. May et al18 described a case of unilateral Leydig cell tumor in a stallion with a descended testes. These authors clinically observed a firm, warm, and mildly painful swelling associated with a bilateral scrotal edema assessed by ultrasound. In the
tract. In: Equine reproduction. Philadelphia: Lea & Febiger; 1992:871–877. 2. Schumacher J. Testicular neoplasia of horse: an underreported condition. Equine Vet J 1999;31:270–272. 3. Brinsko SP. Neoplasia of the male reproductive tract. Vet Clin North Am: Equine Pract 1998;14:517–533. 4. McEntee K. Scrotum, spermatic cord, and testis: proliferative lesions. In: Reproductive pathology of domestic animals. New York: Academic Press; 1990. 5. Leme DP, Papa FO. Cytological identification and quantification of testicular cell types using fine needle aspiration in horses. Equine Vet J 2000;32:444–446. 6. Nachtsheim DA, Scheible FW, Gosnic B. Ultrasonography of testis tumors. J Urol 1983;129:978–981. 7. Lopate C, LeBlanc M, Knottenbelt D. The stallion. In: Equine stud farm medicine and surgery. London: Saunders; 2003; 43–112. 8. Pozor MA. How to evaluate a stallion’s scrotum using ultrasound. In: Proceedings 51st Annual Convention of the American Association of
CM Melo et al Vol 27, No 10 (2007)
Equine Practitioners; Ithaca: International Veterinary Information Service (www.ivis.org), 2005; Document No. P2649.1205. 9. Gelberg HB, McEntee K. Equine testicular interstitial cell tumors. Vet Pathol 1987;24:231–234.
453
14. Baker JR, Leyland A. Histologic survey of tumours of the horse, with particular reference to those of the skin. Vet Rec 1975;96:419–422. 15. Cothcin E, Baker-Smith J. Tumours in horses encountered in abattoir survey. Vet Rec 1975;97:339.
10. Papatsoris AG, Triantafyllidis A, Gekas A, Karamouzis MV,
16. Nielsen SW, Kennedy PC. Tumours in the genital system. In: Tu-
Rosenbaum T. Leydig cell tumor of the testis: new cases and review
mours in domestic animals. Berkeley, CA: University of California
of the current literature. Tumori 2004;99:422–423. 11. Eisenhauer KM, McCue PM, Nayden DK, Osawa Y, Roser JF. Local-
Press; 1990:479–517. 17. Sundberg JP, Burnstein T, Page EH, Kirkha WW, Robinson FR.
ization of aromatase in equine Leydig cells. Domest Anim Endocrinol 1995;11:291–298. 12. Juha´sz J, Nagy P, Kulcsar M, Huszenicza GY. Methods for semen and endocrinological evaluation of the stallion: a review. Acta Vet Brno 2000;69:247–259. 13. Ladds PW. The male genital system. In: Pathology of domestic animals. San Diego: Academic Press, Inc.; 1993:471–531.
Neoplasm of equidae. J Am Vet Med Assoc 1977;170:150–152. 18. May KA, Moll HD, Duncan RB, Pleasant RS, Purswell BJ. Unilateral Leydig cell tumour resulting in acute colic and scrotal swelling in a stallion with descendent testes. Equine Vet J 1999;31:343–345. 19. Hay WP, Baskett A, Gregory CR. Testicular interstitial cell tumour and aplasia of the head of epididymis in a cryptorchid stallion. Equine Vet Educ 1987;9:240–241.
CASE REPORT Bilateral Leydig Cell Tumor in Stallion
C.M. Melo, MS,a F.O. Papa, PhD,a N.C. Prestes, PhD,a M.A. Alvarenga, PhD,a and R. Laufer-Amorim, PhDb ABSTRACT The authors report a rare case of bilateral Leydig cell tumor in a noncryptorchid stallion, describing the gross and microscopic findings. An 8-year-old Appaloosa stallion was examined for a unilateral enlargement of the testis associated with discomfort during palpation and signs of colic. General health conditions were good. Fine-needle aspiration cytology of the testis provided the diagnosis of bilateral Leydig cell tumor. Bilateral orchiectomy was performed and the testes were submitted to histopathologic examination that confirmed the diagnosis of Leydig cell tumor. Keywords: Testis; Interstitial cell tumor; Stallion
INTRODUCTION Testicular tumors are uncommon in stallions, probably because most male horses are castrated at an early age, making it difficult to predict the true incidence of this pathologic condition.1,2 The description of endocrine dysfunction, malignancy, and relationship with cryptorchid testicle is based on other species commonly affected by this condition, such as dogs and humans.2,3 Testicular neoplasms are rare and can be classified as primary, which are the most common tumors, and secondary.1,2 In general, the neoplasias are classified as germinative, comprising seminoma, teratoma, teratocarcinoma, and embrionary carcinoma, or nongerminative: leydigocitoma, Sertoli cell tumor, and lipomas.4 Seminoma is by far the most common testicular tumor of the mature stallion.1,3,4 Fine-needle aspiration biopsy can help to differentiate inflammatory from noninflammatory disease and may allow diagnosis of neoplasia by an experienced cytologist.1 Fine-needle aspiration biopsy has proven to be a simple and minimally invasive diagnostic procedure.5 Testicular ultrasonography is a valuable tool to differentiate intratesticular or extratesticular masses in humans.6 In
stallions, little information is available about use of ultrasonography to determine the type of tumor; consequently a histopathologic examination is necessary to confirm the diagnosis. Tumoral images are variable; thus, ultrasonography will help to identify discrete masses and characterize their density.7 Normal parenchyma should be echoic, whereas focal changes of echogenicity are often associated with a tumor, hematoma, abscess, or local inflammation.8 Gelberg and McEntee9 presented data from nine stallions with Leydig cell tumor. Two of them showed aggressive behavior. Thus, these authors suggested that, in some cases, equine Leydig cell tumor might lead to abnormal steroidogenesis. As reviewed by Papatsoris et al10 in humans, 20% of the cases of Leydig cell tumor demonstrated hormonal activity, including an increase in estradiol and a decrease in testosterone serum levels. Treatment consists of castration using closed techniques, and both testicle and spermatic cord should be removed.1,2 The aim of the current study was to describe the presence of a bilateral Leydig cell tumor in a noncryptorchid stallion and its gross and microscopic findings.
CLINICAL HISTORY
From the Department of Animal Reproduction and Veterinary Radiology—UNESP/Sa˜o Paulo State Universitya; and the Department of Veterinary Clinic—UNESP/Sa˜o Paulo State Universityb; Sa˜o Paulo, Brazil. Reprint requests: C. M. Melo, FMVZ—Department of Animal Reproduction and Veterinary Radiology, Distrito de Rubia˜o Ju´nior s/n8, 18618-000, Botucatu, SP, Brazil. 0737-0806/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2007.08.002
An 8-year-old Appaloosa stallion was presented to the Veterinary Teaching Hospital, Sa˜o Paulo State University, Botucatu, Sao Paulo, for investigation of discomfort during breeding and unilateral enlarged testis for 1 year (Fig. 1). The referring veterinarian had treated the stallion with metronidazole, sulfa, and trimethoprim because of suspicion of an infectious orchitis. He also reported that the stallion had preliminary signs of colic. Standard physical examination of the stallion showed good appearance and all physiologic parameters in order. On testicular palpation, the stallion exhibited pain; testicular texture varied from fibroelastic to firm. Testicles had the following sizes: 12.0 8.0 8.0 cm (left testis) and 6.0 5.0 5.0 cm (right testis) for length, height, and width, respectively. The ultrasound evaluation showed hypoechogenic areas similar to abscess (Fig. 2), which were confirmed at macroscopic examination of testis after surgery (Figs. 3, 4). During semen collection, no abnormal sexual or aggressive behavior was observed and no signs of pain were manifested during erection until ejaculation. Semen analysis showed oligoasthenospermia. Aspirative cytology was performed with a fine needle and a bilateral Leydig cell tumor was diagnosed.
450
Journal of Equine Veterinary Science Vol 27, No 10 (2007)
CM Melo et al Vol 27, No 10 (2007)
Figure 1. Testicular appearance during physical examination.
Figure 2. Ultrasound image of the left and right testes showing the abscess.
According to Eisenhauer et al,11 luteinizing hormone (LH) stimulates production and release of estrogen and testosterone from the Leydig cells. To investigate the influence of the tumor on hormonal profile, blood samples were collected for hormone assays. The human chorionic gonadotropin (hCG) challenge is a test to investigate testicular function of stallions and a diagnostic tool to test the secretory capacity of Leydig cells.12 Because LH promotes spermatogenesis by stimulating Leydig cells to produce testosterone, the stallion received a 6,000-IU hCG intramuscular injection. Jugular blood was collected by venipuncture into Vacutainer tubes before hCG treatment and 48 hours after hCG. The blood was centrifuged for 5 minutes at 1,500g. Serum was stored at –208C until being assayed for concentrations of estrogen and testosterone, performed by BET Labs (Lexington, KY). Hormonal values are presented in Table 1.
451
Figure 3. Left and right testes after orchiectomy.
Figure 4. Left and right testes sectioned after the orchiectomy.
Gross Findings After orchiectomy, testicles were sectioned for macroscopic evaluation. Both testes presented circumscribed masses. On gross examination, both testes presented circumscribed encapsulated tissue with a different aspect from normal parenchyma, containing cystic areas filled with colorless yellow fluid. The tumor was hazel in color and had a smooth texture (Fig. 4).
Microscopic Findings The testes were submitted to histopathology. Histologic examination showed polyhedral cells with granular eosinophilic cytoplasm and vacuolar spaces. The nuclei were small, spherical, and basophilic with scant supporting stromal tissue, presenting a necrotic center with a suppurative inflammatory process (Fig. 5).
CM Melo et al Vol 27, No 10 (2007)
452
Table 1. Estrogen and Testosterone Levels at 0 and 48 Hours after hCG Administration T[0 Hours
T [ 48 Hours
Total estrogen (pg/ml) 338.5 183.9 Testosterone (pg/ml) 2,015.2 3,465.4 Normal values in horses (B.E.T. Laboratories) Total estrogen (pg/ml) Stallion 150 400 pg/ml Gelding < 50 pg/ml Testosterone (pg/ml) Stallion 500 2,000 pg/ml Gelding < 50 pg/ml
Figure 5. Photomicrograph of a section of the testis from a horse with testicular interstitial cell tumor. Notice polyhedral cells with granular eosinophilic cytoplasm and discrete cytoplasmic borders with small, spherical and basophilic nuclei (hematoxylin-eosin, original magnification 400).
current report, the stallion presented some episodes of colic without a clinical cause, similar to the case described by May et al.18 The stallion had no discomfort during semen collection; sperm analysis showed oligoasthenospermia. Papatsoris et al10 suggested that human semen analysis may show oligozoospermia, cryptozoospermia, or azoospermia. Orchiectomy was performed on the day after treatment with hCG, and blood was collected for estrogen and testosterone dosage at 0 and 48 hours after hCG administration. Serum concentration of estrogen was normal both before and 48 hours after hCG injection. Testosterone levels had a significant increase at 48 hours after hCG challenge. However, as expected, a decrease to basal levels (<50 pg/ ml) was observed in testosterone and estrogen levels at 96 hours after removal of testicles. This finding demonstrates that the tumor did not affect responsiveness to LH stimulation by increasing the concentration of testosterone. The use of fine-needle aspiration biopsy was as efficient as histologic evaluation, which may lead to the conclusion that this examination should be performed in stallions with a history of infertility, especially associated with an abnormal testis. Finally, the current case was the first report of a bilateral Leydig cell tumor in a stallion with both descended testes. The current report also evaluates endocrinology of the tumor, which has not been described by other authors.3,9,18,19 The evaluation and description of Leydig cell tumors in stallions should be performed whenever it is possible; it can contribute to our knowledge and understanding of this tumor, especially endocrine evaluations. ACKNOWLEDGMENTS The authors thank BET Labs for the hormonal evaluation.
REFERENCES 1. Schumacher J, Varner DD. Neoplasia of the stallion’s reproductive
DISCUSSION AND CONCLUSION Primary testicular tumors are relatively common in old dogs, less common in aged bulls, and rare in other species. These tumors can be of three types: interstitial Leydig cell, Sertoli cell, and germinative cells. These neoplasms are observed in sexually mature animals or older males, as in interstitial cell tumors in dog.13 Testicular neoplasms are rare in horses,14-17 although it is difficult to predict or estimate their occurrence, because most animals are castrated young.3 Cryptorchidism may be a contributing factor in the formation of Leydig cell tumors in stallions. Gelberg and McEntee9 observed the occurrence of this tumor in the abdominal testes of 8 of 9 cryptorchid stallions. May et al18 described a case of unilateral Leydig cell tumor in a stallion with a descended testes. These authors clinically observed a firm, warm, and mildly painful swelling associated with a bilateral scrotal edema assessed by ultrasound. In the
tract. In: Equine reproduction. Philadelphia: Lea & Febiger; 1992:871–877. 2. Schumacher J. Testicular neoplasia of horse: an underreported condition. Equine Vet J 1999;31:270–272. 3. Brinsko SP. Neoplasia of the male reproductive tract. Vet Clin North Am: Equine Pract 1998;14:517–533. 4. McEntee K. Scrotum, spermatic cord, and testis: proliferative lesions. In: Reproductive pathology of domestic animals. New York: Academic Press; 1990. 5. Leme DP, Papa FO. Cytological identification and quantification of testicular cell types using fine needle aspiration in horses. Equine Vet J 2000;32:444–446. 6. Nachtsheim DA, Scheible FW, Gosnic B. Ultrasonography of testis tumors. J Urol 1983;129:978–981. 7. Lopate C, LeBlanc M, Knottenbelt D. The stallion. In: Equine stud farm medicine and surgery. London: Saunders; 2003; 43–112. 8. Pozor MA. How to evaluate a stallion’s scrotum using ultrasound. In: Proceedings 51st Annual Convention of the American Association of
CM Melo et al Vol 27, No 10 (2007)
Equine Practitioners; Ithaca: International Veterinary Information Service (www.ivis.org), 2005; Document No. P2649.1205. 9. Gelberg HB, McEntee K. Equine testicular interstitial cell tumors. Vet Pathol 1987;24:231–234.
453
14. Baker JR, Leyland A. Histologic survey of tumours of the horse, with particular reference to those of the skin. Vet Rec 1975;96:419–422. 15. Cothcin E, Baker-Smith J. Tumours in horses encountered in abattoir survey. Vet Rec 1975;97:339.
10. Papatsoris AG, Triantafyllidis A, Gekas A, Karamouzis MV,
16. Nielsen SW, Kennedy PC. Tumours in the genital system. In: Tu-
Rosenbaum T. Leydig cell tumor of the testis: new cases and review
mours in domestic animals. Berkeley, CA: University of California
of the current literature. Tumori 2004;99:422–423. 11. Eisenhauer KM, McCue PM, Nayden DK, Osawa Y, Roser JF. Local-
Press; 1990:479–517. 17. Sundberg JP, Burnstein T, Page EH, Kirkha WW, Robinson FR.
ization of aromatase in equine Leydig cells. Domest Anim Endocrinol 1995;11:291–298. 12. Juha´sz J, Nagy P, Kulcsar M, Huszenicza GY. Methods for semen and endocrinological evaluation of the stallion: a review. Acta Vet Brno 2000;69:247–259. 13. Ladds PW. The male genital system. In: Pathology of domestic animals. San Diego: Academic Press, Inc.; 1993:471–531.
Neoplasm of equidae. J Am Vet Med Assoc 1977;170:150–152. 18. May KA, Moll HD, Duncan RB, Pleasant RS, Purswell BJ. Unilateral Leydig cell tumour resulting in acute colic and scrotal swelling in a stallion with descendent testes. Equine Vet J 1999;31:343–345. 19. Hay WP, Baskett A, Gregory CR. Testicular interstitial cell tumour and aplasia of the head of epididymis in a cryptorchid stallion. Equine Vet Educ 1987;9:240–241.